1. Field of the Invention
This invention relates to a spark plug and a method of making the spark plug in which a spark gap is provided between an elevational side of a center electrode axially extended in an tubular insulator and one end of a first spark-resistant noble metal tip secured to an outer electrode.
2. Description of Prior Art
In a spark plug for an internal combustion engine, the following techniques have been used to secure a spark-erosion resistant noble metal or noble metal alloy tip to an outer electrode.
(i) As a noble metal tip 121, a thin layer of Pt--Ir or Pt--Ni alloy is welded to one end of an outer electrode 120 in a manner to oppose an elevational side of a center electrode 110 of a spark plug 100 as shown in FIG. 16. PA0 (ii) A noble metal elongation 122 is secured to one end of the outer electrode 120 by means of argon welding in a manner to oppose an elevational side of the center electrode 110 of the spark plug 100 as shown in FIG. 17. PA0 (iii) A noble metal tip 123 is welded to an upper side 120a of the outer electrode 120 in a manner to oppose a front end 110a of the center electrode 110 as shown in FIG. 18. PA0 (iv) A short pedestal 220 is placed on a front end 210a of a metallic shell 210 in a direction according to an extension of the metallic shell 210. Then a spark-resistant noble metal tip 230 is prepared from Pt--Ir or Pt--Ni alloy, and secured to one end of an outer electrode 240. Thereafter, the outer electrode 240 is secured to the short pedestal 220 by means of electric resistance welding. During the welding procedure, a spacer 260 is used to provide a spark gap between the tip 230 and a center electrode 250 as shown in FIG. 19.
In the technique (i), a variation may be induced in a lateral arm 120b of the outer electrode 120 to deteriorate its dimensional accuracy upon bending the outer electrode 120 into the L-shaped configuration after welding the thin layer of noble metal tip 121 to one end of the outer electrode 120. When the tip 121 is welded to the outer electrode 120 once the outer electrode is bent into the L-shaped configuration, it is troublesome to weld the tip 121 so as to only deteriorate mass production since one end of the outer electrode 120 is located to oppose the elevational side of the center electrode 110. The thin layer of the tip 121 shortens a distance between the one end of the outer electrode and the elevational side of the center electrode 110 so as to worsen the ignitibility due to an increased flame-extinguishing effect.
In the technique (ii), a variation may be induced in the lateral arm 120b of the outer electrode 120 to deteriorate its dimensional accuracy upon bending the outer electrode 120 into the L-shaped configuration, after argon welding the noble metal elongation 122 to one end of the outer electrode 120. When the noble metal elongation 122 is welded to the outer electrode 120 after bending the L-shaped configuration, it is troublesome to thermally weld the elongation 122 since one end of the outer electrode 120 is located to oppose the elevational side of the center electrode 110. Consequently, mass production is difficult.
In the technique (iii), the lateral arm 120b of the outer electrode 120 is likely to increase its weight unilaterally since a total length (L1+L2) of the outer electrode 120 is longer than in a lateral discharge type spark plug, in addition to the fact that the noble metal tip 123 is welded to the upper side of the outer electrode 120. This makes it possible to break down the outer electrode when exposed to persistent vibration. The technique (iii) also has an unfavorable tendency to require a high discharge voltage when the positive polarity voltage is applied to the center electrode.
In the manufacturing method (iv), an excessive pressure may be applied to the pedestal 220 so as to unfavorably deform the pedestal 220 due to the electric resistance welding upon securing the outer electrode 240 to the pedestal 220. The deformed pedestal causes a change in the height of the outer electrode 240 so as to adversely affect the performance of the spark plug although the required spark gap is maintained.
Therefore, it is an object of the invention provide a spark plug which is capable of preventing the outer electrode from being broken down when exposed persistent vibration, and readily welding the spark-erosion resistant noble metal tip while maintaining a good ignitibility with less flame extinguishing effect.
It is another object of the invention to provide a method of making a high quality spark plug which is capable of obviating the necessity of adjusting a spark gap after the spark-erosion resistant noble metal tip is secured to the outer electrode in a spark plug in which the spark gap is provided between an elevational side of a center electrode axially extended along a tubular insulator and one end of a first spark-erosion resistant noble metal tip secured to an outer electrode.